Advertisement

Contraceptive use and discontinuation: Findings from the contraceptive history, initiation, and choice study

Published:April 24, 2006DOI:https://doi.org/10.1016/j.ajog.2005.11.039

      Objective

      The purpose of this study is to provide insight on the continuing high rate of unintended pregnancy among adult women.

      Study design

      Contracepting women were recruited while they waited for primary care appointments. A total of 369 completed the baseline questionnaire, and 145 oral contraceptive (OC) users were enrolled in a 5-week, diary-based study of adherence and sexual activity.

      Results

      Most women who reported having discontinued OCs did so because of medical side effects, and most had switched to less effective methods. Among OC users, 26.4% had sexual intercourse on days they missed pills just before or after their placebo week. Nonadherence did not differ by socioeconomic factors or obesity.

      Conclusion

      Clinicians may need to encourage their patients to discuss their reasons for wanting to discontinue the use of an effective contraceptive method and assist them with their concerns or to switch to other effective methods to protect themselves from unintended pregnancy.

      Key words

      The National Survey of Family Growth (NSFG) provides researchers with important information on factors affecting pregnancy and women's health among women age 15 to 44 years. Since 1982, in-depth interviews conducted by NSFG staff have increased the knowledge on a number of issues, including the use of contraception.
      • Mosher W.D.
      • Martinez G.M.
      • Chandra A.
      • Abma J.C.
      • Wilson S.J.
      Use of contraception and use of family planning services in the United States: 1982-2002.
      Between 1995 and the latest NSFG in 2002, the percentage of sexually active women not using contraception increased from 5.2% in 1995 to 7.4%. This represents an apparent increase of 1.43 million women at risk for unintended pregnancies.
      • Mosher W.D.
      • Martinez G.M.
      • Chandra A.
      • Abma J.C.
      • Wilson S.J.
      Use of contraception and use of family planning services in the United States: 1982-2002.
      Oral contraceptives (OCs) are the most commonly used non-permanent method of contraception in the United States.
      • Mosher W.D.
      • Martinez G.M.
      • Chandra A.
      • Abma J.C.
      • Wilson S.J.
      Use of contraception and use of family planning services in the United States: 1982-2002.
      Among OC users, adherence to the dosage regimen and frequency of intercourse are major determinants of unintended pregnancy.
      • Trussel J.
      • Kowal D.
      The essentials of contraception.
      Though integral to studies of fertility and contraceptive effectiveness, including recent studies of a possible obesity-oral contraceptive failure association,
      • Vessey M.
      • Painter R.
      Oral contraceptive failures and body weight: findings in a large cohort study.
      • Holt V.L.
      • Cushing-Haugen K.L.
      • Daling J.R.
      Body weight and risk of oral contraceptive failure.
      • Holt V.L.
      • Scholes D.
      • Wicklund K.G.
      • Cushing-Haugen K.L.
      • Daling J.R.
      Body mass index, weight, and oral contraceptive failure risk.
      • Brunner L.R.
      • Hogue C.J.
      The role of body weight in oral contraceptive failure: findings from the 1995 National Survey of Family Growth.
      information on these determinants is limited. The purpose of the Contraceptive History, Initiation, and Choice (CHIC) Study is 2-fold: first, to provide more insight into the demographic and lifestyle characteristics associated with contraceptive use and discontinuation, and second, to determine whether body mass index (BMI) is associated with adherence or frequency of sexual intercourse among OC users.

      Material and methods

      Study population and design

      The CHIC Study was conducted at a suburban Family Medicine clinic in the Atlanta area from May to November 2004. The clinic is affiliated with Emory University and serves as the primary training area for residents of the Family Medicine Residency Program. The CHIC Study protocol was approved by the Emory University Institutional Review Board on April 24, 2004. Women between the ages of 18 and 45 years who were using any method of birth control were approached while waiting for their appointment (n = 413), and those who agreed to participate signed an informed consent form and filled out a short, baseline questionnaire (n = 369 by September 2004).
      OC users were invited to participate in a longitudinal study, in which the women were requested to fill out five 1-week diaries, in which they recorded daily information on whether they took their pill, the exact time they took their pill, and if they engaged in sexual intercourse. Women who agreed to participate received a coupon for a free movie rental for each diary they completed and mailed back to the study personnel. After September 2004, recruitment focused on enrolling OC users in the longitudinal portion of the study to increase sample size. Thus, during the remainder of the study period, only OC users between the ages of 18 and 45 years were asked to fill out the baseline questionnaire. An additional 12 women were recruited for the longitudinal study during this time. Of the 158 OC users, 13 (8.2%) declined to take part in the longitudinal study. Of the 145 women who agreed to participate in the longitudinal portion of the study, 98 (67.6%) returned at least 1 diary. Nonrespondents were contacted by telephone up to 3 times, and finally by mail. Of the 47 nonrespondents, 6 indicated discontinuation of OCs as the reason for not participating, whereas no information was available for the remaining women. Nonrespondents and women who declined to participate were less educated (25.0% attended graduate school vs 40.8% of respondents) and more likely to be of black race/ethnicity as compared with respondents (36.7% vs 19.4%). Respondents and nonrespondents did not differ by age (65.3% vs 65.0% <30 years), weight (44.9% vs 42.3% <150 lbs), or marital status (54.1% vs 56.7% single).

      Measurement of covariates

      The baseline questionnaire collected information on age, race/ethnicity, marital status, education, dual method use (use of an additional contraceptive method), prior contraceptive method use, and reason for discontinuation of a contraceptive method. Diaries collected information on the following variables: income, smoking, reason(s) for using OCs, length of time using OCs, parity, future birth intention, history of abortion, and whether the diary week was typical in terms of family, work, and social responsibilities. Information on height and weight were abstracted from medical records.

      Definition of adherence and sexual intercourse outcomes

      Adherence during a week was defined as missing no pills during an active week. Diary information was used to create a dichotomous sexual intercourse variable for each week.

      Analysis

      Summary statistics of the demographic and lifestyle characteristics of participants were calculated. For contraceptive method categories with sufficient numbers, characteristics of these specific users were further investigated. Unadjusted odds ratios (ORs) and 95% CIs were obtained to examine the association between BMI and the adherence with an OC regimen and frequency of intercourse outcomes. For the adherence analyses, weeks during which women took placebo pills were excluded. Because the data include repeated measures for each woman, a generalized estimating equations approach was used to carry out logistic regression for correlated responses for the weekly adherence and sexual intercourse outcomes.
      • Kleinbaum D.G.
      • Klein M.
      Logistic regression: a self-learning text.
      Multivariate logistic regression for correlated responses was used to further explore the relationship between obesity and the adherence and sexual intercourse outcomes. Only those variables that changed point estimates by 10% or more were included in the final model as confounders.
      • Maldonado G.
      • Greenland S.
      Simulation study of confounder selection strategies.
      All analyses were performed with the SAS System for Windows Version 8.2 (SAS Institute, Cary, NC).

      Results

      Of the 369 women who were initially recruited into the CHIC Study, the majority were 26 to 35 years old (mean: 29.3), single, and highly educated (Table I). Most participants were of race/ethnicity white or black (48.8% and 40.9%, respectively) and nearly half had BMIs in the overweight or obese range (mean BMI = 27.2). The most popular method of contraception was OCs (39.6%), followed by male condoms (21.4%), and tubal ligation (7.6%). The majority of women were not dual method users (75.6%); however, among those women who did use an additional method, male condoms were the most popular method.
      Table ICharacteristics of women who participated in the CHIC Study, 2004
      CharacteristicNumber (%)
      Age (y)
       18-25116 (31.4)
       26-35188 (51.0)
       36-4565 (17.6)
      Education
       High school30 (8.1)
       Some college113 (30.6)
       College graduate124 (33.6)
       Graduate school102 (27.6)
      Marital status
       Married117 (31.7)
       Divorced, separated, widowed38 (10.3)
       Living with partner40 (10.8)
       Single174 (47.2)
      Race/ethnicity
       Asian21 (5.7)
       Black151 (40.9)
       Hispanic17 (4.6)
       White180 (48.8)
      BMI
       <2031 (8.4)
       20-24.9157 (42.6)
       25-29.984 (22.8)
       ≥3097 (26.3)
      Main current method of contraception
       Oral contraceptives146 (39.6)
       Male condoms79 (21.4)
       Female condoms2 (0.5)
       Injectables18 (4.9)
       Transdermal patch13 (3.5)
       IUD13 (3.5)
       Diaphragm, cap, sponge1 (0.3)
       Withdrawal21 (5.7)
       Rhythm method5 (1.4)
       Abstinence18 (4.9)
       Tubal ligation28 (7.6)
       Vasectomy17 (4.6)
       Other8 (2.2)
      Secondary method of contraception
       None279 (75.6)
       Male condoms66 (17.9)
       Withdrawal18 (4.9)
       Rhythm method6 (1.6)
      Ninety-one women had never used another form of birth control before beginning their current method. Many of these women chose effective methods to protect themselves from unintended pregnancy: 52.8% OCs; 24.2% male condoms; 4.4% injectables, transdermal patch, or vaginal ring; 2.2% intrauterine device (IUD); 2.2% withdrawal; and 14.2% diaphragm or other unspecified methods.
      Approximately two-fifths (42%) of women had experience with condoms. Among women who reported using condoms before starting their current method of contraception (n = 76), women discontinued the use of this method for the following reasons: 22.4% not having sex, 6.6% trying to get pregnant, and 71.1% for other reasons. Of those women who elaborated on their reasons for discontinuing the use of condoms, 22 indicated they wanted to switch to an easier or more reliable method, 12 were in a monogamous relationship, and 6 thought that condoms were uncomfortable and interfered with their sex life. Other reasons for discontinuation included vaginal irritation and latex allergies. The vast majority of these women indicated that they were now using a more effective method of birth control (71.1% OCs, 6.6% injectables or transdermal patch).
      Two-thirds of the women had experience with OCs. Among the 128 who reported using OCs just before switching to their current method, 22.7% discontinued the use of OCs because they were not having sex, 14.1% were trying to get pregnant, and the remaining 63.3% discontinued OCs for other reasons. Among those women who elaborated on what these other reasons for discontinuation were, 12 indicated that either they or their partners underwent a sterilization procedure, 10 experienced nausea while taking OCs, 6 had episodes of depression, 6 wanted a “break from hormones,” 5 had weight gain, 5 had headaches while using OCs, and 5 became pregnant while using OCs. Other reasons for discontinuation included yeast infections, issues with insurance coverage, adherence problems, development of hypertension, and breakthrough bleeding. Many of these women changed to another reliable form of birth control after discontinuing OCs (32.8% a hormonal method, 11.7% sterilization). However, 10.2% started to use the withdrawal method as their primary method of birth control.
      The majority of current OC users were older than 26 years, highly educated, single, and white (Table II). Nearly 60% of current OC users had BMIs less than 25 and 65.8% did not use a method in addition to OCs. Women who reported currently using condoms as their primary method of birth control were similar to current OC users in that the majority were older than 26 years, highly educated, and single. However, most condom users were black and had BMIs in the overweight or obese range.
      Table IICharacteristics of women according to their current method of contraception, CHIC Study, 2004
      CharacteristicOral contraceptivesCondoms
      Condom group includes male and female condoms.
      Age (y)
       18-2552 (35.6)30 (37.0)
       26-3583 (56.9)34 (42.0)
       36-4511 (7.5)17 (21.0)
      Education
       High school10 (6.9)9 (11.1)
       Some college29 (19.9)27 (33.3)
       College graduate56 (38.4)23 (28.4)
       Graduate school51 (34.9)22 (27.2)
      Marital status
       Married43 (29.5)25 (30.9)
       Divorced, separated, widowed7 (4.8)8 (9.9)
       Living with partner23 (15.8)4 (4.9)
       Single73 (50.0)44 (54.3)
      Race/ethnicity
       Asian9 (6.2)3 (3.7)
       Black41 (28.1)44 (54.3)
       Hispanic7 (4.8)2 (2.5)
       White89 (61.0)32 (39.5)
      BMI
       <2013 (8.9)8 (9.9)
       20-24.975 (51.4)24 (29.6)
       25-29.925 (17.1)26 (32.1)
       ≥3033 (22.6)23 (28.4)
      Secondary method
       None96 (65.8)66 (81.5)
       Male condoms43 (29.5)1 (1.2)
       Withdrawal6 (4.1)11 (13.6)
       Rhythm method1 (0.7)3 (3.7)
      Prior main method
       No prior method48 (32.9)22 (27.2)
       Oral contraceptives17 (11.6)40 (49.4)
       Condoms54 (37.0)3 (3.7)
       Other hormonal
      Other hormonal group includes injectables, transdermal patch, and vaginal ring.
      16 (11.0)10 (12.4)
       IUD2 (1.4)2 (2.5)
       Withdrawal/rhythm7 (4.8)2 (2.5)
       Other
      Other group includes diaphragm, foam, and unspecified “other” responses.
      2 (1.4)2 (2.5)
      Condom group includes male and female condoms.
      Other hormonal group includes injectables, transdermal patch, and vaginal ring.
      Other group includes diaphragm, foam, and unspecified “other” responses.
      The 98 current OC users who participated in the longitudinal portion of the CHIC Study returned a total of 413 diaries, with 70.4% returning all 5 diaries. During the placebo weeks, women reported an average of 3.83 days of bleeding. The majority (84.0%) of weeks during which women were taking active pills were classified as adherent. There was no clear association between BMI and adherence in the unadjusted model (Table III; P for trend = .60). The multivariate model of BMI and adherence also found no association after adjustment for race/ethnicity, marital status, education, parity, income, and use of OCs to regulate menstrual cycles (P for trend = .61).
      Table IIIUnadjusted and multivariate-adjusted association between BMI and adherence among oral contraceptive users who participated in the CHIC Study, 2004
      CharacteristicUnadjusted OR (95% CI)Multivariate-adjusted
      BMI-adherence association adjusted for race/ethnicity, marital status, income, parity, education, and using OCs to regulate menstrual cycle.
      OR (95% CI)
      BMI
       <201.21 (0.20-7.26)0.93 (0.24-3.60)
       20-24.91.00 (referent)1.00 (referent)
       25-29.91.21 (0.27-5.49)0.84 (0.18-3.88)
       ≥301.15 (0.45-2.98)1.09 (0.32-3.68)
      P value for trend = .60P value for trend = .61
      BMI-adherence association adjusted for race/ethnicity, marital status, income, parity, education, and using OCs to regulate menstrual cycle.
      On average, women had sexual intercourse 1.5 times a week. Of note, 26.4% of women had sexual intercourse on days they missed pills just before or after their placebo week. Women who miss pills immediately before or after the placebo week resume some normal follicular development
      • Killick S.R.
      • Bancroft K.
      • Oelbaum S.
      • Morris J.
      • Elstein M.
      Extending the duration of the pill-free interval during combined oral contraception.
      • Landgren B.M.
      • Diczfalusy E.
      Hormonal consequences of missing the pill during the first two days of three consecutive artificial cycles.
      • Letterie G.S.
      • Chow B.E.
      Effect of “missed” pills on oral contraceptive effectiveness.
      • Molloy B.G.
      • Coulson K.A.
      • Lee J.M.
      • Watters J.K.
      “Missed pill” conception: fact or fiction?.
      and may be at risk of pregnancy.
      • Guillebaud J.
      Contraception: your questions answered.
      In the unadjusted model, there was no clear association between BMI and sexual intercourse during a week (Table IV; P for trend = .29). After adjustment for race/ethnicity, marital status, income, parity, and use of OCs to regulate menstrual cycles or alleviate cramps, there was still no indication that BMI was associated with sexual intercourse (P for trend = .37).
      Table IVUnadjusted and multivariate-adjusted association between BMI and sexual intercourse among oral contraceptive users who participated in the CHIC Study, 2004
      CharacteristicUnadjusted OR (95% CI)Multivariate-adjusted
      BMI-sexual intercourse association adjusted for race/ethnicity, marital status, income, parity, using OCs to alleviate cramps and using OCs to regulate menstrual cycle.
      OR (95% CI)
      BMI
       <201.13 (0.20-7.26)1.27 (0.35-4.58)
       20-24.91.00 (referent)1.00 (referent)
       25-29.91.12 (0.27-5.49)1.84 (0.64-5.32)
       ≥300.75 (0.45-2.98)1.22 (0.53-2.81)
      P value for trend = .29P value for trend = .37
      BMI-sexual intercourse association adjusted for race/ethnicity, marital status, income, parity, using OCs to alleviate cramps and using OCs to regulate menstrual cycle.

      Comment

      In the CHIC Study, we found that among women age 18 to 44 years using birth control, the most popular methods of contraception were OCs, male condoms, and tubal ligation. The majority of women who had stopped using OCs as their main method of contraception had discontinued because of medical side effects, whereas the majority of women who had discontinued the use of condoms had ceased because they desired a more effective and reliable form of birth control. In addition, unadjusted and adjusted models of BMI and adherence to an OC regimen and frequency of intercourse showed no clear association and all included the null value.
      Our findings parallel those of the 2002 NSFG, in which the 3 most popular methods of contraception among women between the ages of 15 and 44 years who were using a method of birth control were OCs, female sterilization, and male condoms.
      • Mosher W.D.
      • Martinez G.M.
      • Chandra A.
      • Abma J.C.
      • Wilson S.J.
      Use of contraception and use of family planning services in the United States: 1982-2002.
      The 2002 NSFG found that among women who reported using contraception, more women were using effective methods of contraception compared with 1995 survey results. However, few women relied on other, effective methods of birth control such as injectables, the transdermal patch, and the IUD; this finding is mirrored in our study population. Alarmingly, both the CHIC Study and the 2002 NSFG revealed that withdrawal was more popular than either the transdermal patch or IUD.
      Studies conducted in the United States and Europe to investigate reasons for the discontinuation of OCs found that the majority of women stop using the method because of side effects.
      • Rosenberg M.J.
      • Waugh M.S.
      • Meehan T.E.
      Use and misuse of oral contraceptives: risk indicators for poor pill taking and discontinuation.
      • Rosenberg M.J.
      • Waugh M.S.
      Oral contraceptive discontinuation: a prospective evaluation of frequency and reasons.
      Some of the side effects cited in these studies that were also indicated by the participants of the CHIC Study included nausea, breakthrough bleeding, weight gain, headaches, and mood changes. In the study conducted in the United States, women also cited difficulties in adherence, concerns about safety, and cost as additional reasons for discontinuation.
      • Rosenberg M.J.
      • Waugh M.S.
      Oral contraceptive discontinuation: a prospective evaluation of frequency and reasons.
      Unfortunately, more than half of the women in the CHIC Study who discontinued the use of OCs switched to a less effective contraceptive method.
      As previously mentioned, some recent studies have suggested that obesity increases a woman's risk of OC failure.
      • Holt V.L.
      • Cushing-Haugen K.L.
      • Daling J.R.
      Body weight and risk of oral contraceptive failure.
      • Holt V.L.
      • Scholes D.
      • Wicklund K.G.
      • Cushing-Haugen K.L.
      • Daling J.R.
      Body mass index, weight, and oral contraceptive failure risk.
      These studies lacked complete information on women's adherence to an OC regimen or frequency of intercourse. The longitudinal portion of the CHIC Study found no association between BMI and either of these variables, suggesting that if there is a decreased efficacy of OCs in obese women, it is likely unrelated to less adherence or more sexual intercourse in this population.
      The current study has a number of strengths. Nearly 90% of eligible women agreed to complete the baseline questionnaire. Women who indicated that they discontinued the use of a method because of medical or other reasons were given the option of elaborating on these reasons. Many women took advantage of this option and provided detailed reasons for their discontinuation. Such information is not readily available in national surveys of contraceptive behaviors, including the NSFG.
      In addition, the use of diaries allowed for prospective collection of data on both adherence and frequency of sexual intercourse. Prior studies of adherence have relied on women recalling the number of pills missed during their last pill pack, but the CHIC Study collected this information from women on a daily basis. In a small study that compared self-reported OC compliance with compliance information obtained using an electronic device, women underreported the number of pills they missed.
      • Potter L.
      • Oakley D.
      • de Leon-Wong E.
      • Canamar R.
      Measuring compliance among oral contraceptive users.
      There is therefore the possibility that the adherence information suffers from nondifferential misclassification. However, the diary format provides detailed information on adherence and any resulting misclassification would most likely bias associations toward the null.
      Nondifferential misclassification of sexual intercourse is also possible, but not likely to be substantial. Frequency of sexual intercourse obtained retrospectively versus prospectively is greater, suggesting that women tend to over report their frequency when asked to recall such information.
      • Hornsby P.P.
      • Wilcox A.J.
      Validity of questionnaire information on frequency of coitus.
      • Feldblum P.J.
      • Weir S.S.
      Retrospective versus prospective estimates of number of sexual partners.
      • Steiner M.J.
      • Hertz-Picciotto I.
      • Taylor D.
      • Schoenbach V.
      • Wheeless A.
      Retrospective vs. prospective coital frequency and menstrual cycle length in a contraceptive effectiveness trial.
      In addition, a study that obtained information on daily sexual intercourse by using diaries or a phone-in regimen did not find that the phone-in regimen improved the validity of the data.
      • Hays M.A.
      • Irsula B.
      • McMullen S.L.
      • Feldblum P.J.
      A comparison of three daily coital diary designs and a phone-in regimen.
      The biggest limitation to the CHIC Study is the small sample size and the substantial initial loss to follow-up for the longitudinal portion of the study. Although 145 women agreed to participate in the study, only 98 women returned at least 1 diary. Attempts were made to minimize losses to follow-up by providing an incentive and contacting nonrespondents via both telephone and mail. It was possible to compare respondents and nonrespondents on a number of variables collected on the baseline questionnaire. Although nonrespondents were less educated and more likely to be of race/ethnicity black compared with respondents, they did not differ by age, weight, or marital status. It is possible that a selection bias occurred and the observed associations for the adherence and frequency of sexual intercourse variables may be over or underestimates of the true associations.
      Although a number of women use effective means of birth control such as OCs, few women take advantage of other effective methods including the transdermal patch and IUD. Though it is promising that the majority of women who discontinue the use of condoms or initiate the use of contraception for the first time choose effective methods of contraception, it is alarming that the vast majority of women who discontinue the use of OCs switch to a less effective method of contraception. In our study, approximately 10% of former OC users switched to withdrawal as their primary method of contraception. Perhaps clinicians need to engage their patients in more discussion on why they are discontinuing the use of an effective method of contraception, such as OCs. For many of the reasons indicated, a different dosage could alleviate the side effects or concerns while still providing effective protection against unintended pregnancy. Conversely, clinicians could discuss the advantages of other effective methods including the transdermal patch, IUD, vaginal ring, or injectables so that women would not choose less reliable methods of birth control and put themselves at risk of unintended pregnancy. If a patient still expressed interest in using a less effective method such as withdrawal, clinicians could use this opportunity to educate not only the patient, but also her partner, on how to correctly and consistently use this method. Regardless of the type of contraception a woman chooses to use, clinicians may also want to discuss how to use and obtain emergency contraception because even the most effective methods cannot prevent unintended pregnancy if they are not used correctly and consistently. Expanded knowledge on discontinuation of contraceptive methods, as well as adherence to a contraceptive regimen and frequency of sexual intercourse, can assist researchers of fertility, contraceptive effectiveness, and sexually transmitted infections.

      References

        • Mosher W.D.
        • Martinez G.M.
        • Chandra A.
        • Abma J.C.
        • Wilson S.J.
        Use of contraception and use of family planning services in the United States: 1982-2002.
        National Center for Health Statistics, Hyattsville (MD)2004 (Advance data from vital and health statistics no. 350)
        • Trussel J.
        • Kowal D.
        The essentials of contraception.
        in: Hatcher R.A. Trussel J. Contraceptive technology. Ardent Media Inc, New York (NY)1998: 211-248
        • Vessey M.
        • Painter R.
        Oral contraceptive failures and body weight: findings in a large cohort study.
        J Fam Plann Reprod Health Care. 2001; 27: 90-91
        • Holt V.L.
        • Cushing-Haugen K.L.
        • Daling J.R.
        Body weight and risk of oral contraceptive failure.
        Obstet Gynecol. 2002; 99: 820-827
        • Holt V.L.
        • Scholes D.
        • Wicklund K.G.
        • Cushing-Haugen K.L.
        • Daling J.R.
        Body mass index, weight, and oral contraceptive failure risk.
        Obstet Gynecol. 2005; 104: 46-52
        • Brunner L.R.
        • Hogue C.J.
        The role of body weight in oral contraceptive failure: findings from the 1995 National Survey of Family Growth.
        Ann Epidemil. 2005; 15: 492-499
        • Kleinbaum D.G.
        • Klein M.
        Logistic regression: a self-learning text.
        Springer, New York (NY)2002
        • Maldonado G.
        • Greenland S.
        Simulation study of confounder selection strategies.
        Am J Epidemiol. 1993; 138: 923-936
        • Killick S.R.
        • Bancroft K.
        • Oelbaum S.
        • Morris J.
        • Elstein M.
        Extending the duration of the pill-free interval during combined oral contraception.
        Adv Contracept. 1990; 6: 33-40
        • Landgren B.M.
        • Diczfalusy E.
        Hormonal consequences of missing the pill during the first two days of three consecutive artificial cycles.
        Contraception. 1984; 29: 437-446
        • Letterie G.S.
        • Chow B.E.
        Effect of “missed” pills on oral contraceptive effectiveness.
        Obstet Gynecol. 1992; 79: 979-982
        • Molloy B.G.
        • Coulson K.A.
        • Lee J.M.
        • Watters J.K.
        “Missed pill” conception: fact or fiction?.
        BMJ. 1985; 290: 1474-1475
        • Guillebaud J.
        Contraception: your questions answered.
        Churchill Livingstone, London1993
        • Rosenberg M.J.
        • Waugh M.S.
        • Meehan T.E.
        Use and misuse of oral contraceptives: risk indicators for poor pill taking and discontinuation.
        Contraception. 1995; 51: 283-288
        • Rosenberg M.J.
        • Waugh M.S.
        Oral contraceptive discontinuation: a prospective evaluation of frequency and reasons.
        Am J Obstet Gynecol. 1998; 179: 577-582
        • Potter L.
        • Oakley D.
        • de Leon-Wong E.
        • Canamar R.
        Measuring compliance among oral contraceptive users.
        Fam Plann Perspect. 1996; 28: 154-158
        • Hornsby P.P.
        • Wilcox A.J.
        Validity of questionnaire information on frequency of coitus.
        Am J Epidemiol. 1989; 130: 94-99
        • Feldblum P.J.
        • Weir S.S.
        Retrospective versus prospective estimates of number of sexual partners.
        AIDS. 1995; 9: 1294-1295
        • Steiner M.J.
        • Hertz-Picciotto I.
        • Taylor D.
        • Schoenbach V.
        • Wheeless A.
        Retrospective vs. prospective coital frequency and menstrual cycle length in a contraceptive effectiveness trial.
        Ann Epidemiol. 2001; 11: 428-433
        • Hays M.A.
        • Irsula B.
        • McMullen S.L.
        • Feldblum P.J.
        A comparison of three daily coital diary designs and a phone-in regimen.
        Contraception. 2001; 63: 159-166